Method for the Preparation of 2&#39;-Deoxy-2&#39;,2&#39;-Difluorocytidine

ABSTRACT

This invention relates to an improved method for stereoselectively preparing 2¢¥-deoxy-2¢¥,2¢¥-difluorocytidine of formula (I), which comprises the steps of reacting a 1-halo ribofuranose compound of formula (III) with a nucleobase of formula (IV) in a solvent to obtain a nucleoside of formula (II) with removing the silyl halide of formula (V) produced during the reaction; and deprotecting the nucleoside of formula (II) to obtain 2¢¥-deoxy-2¢¥,2¢¥-difluorocytidine of formula (I).

FIELD OF THE INVENTION

The present invention relates to a method for stereoselectivelypreparing 2′-deoxy-2′,2′-difluorocytidine.

DESCRIPTION OF THE PRIOR ART

2′-Deoxy-2′,2′-difluorocytidine (Gemcitabine) of formula (I) has acytosine nucleobase stereochemically-oriented to β-direction at the1-position of the ribofuranose backbone, and is effective for treatingvarious cancers such as non-small cell lung (NSCLC), pancreatic,bladder, breast or ovarian cancers.

Gemcitabine can be conventionally prepared from a lactol compound asshown in Reaction Scheme 1 via an activated ribofluranose intermediatehaving a reactive leaving group.

wherein, P¹ is a hydroxyl protecting group and L is a leaving group.

Examples of the activated ribofuranose intermediate for glycosylationare 1-sulfonate ribofuranose such as α-methanesulfonate ribofuranose and1-halo ribufuiranose.

The α-methanesulfonate ribofuranose may be reacted with a nucleobase tocarry out stereoselective glycosylation to obtain the desiredβ-nucleosides in a high yield (See U.S. Pat. Nos. 5,371,210, 5,401,838,5,426,183, 5,594,124 and 5,606,048 and EP Patent No. 577303). However,so as to produce α-methanesulfonate ribofuranose in a high ratio ascompared with β-methanesulfonate ribofuranose, it is required to acryogenic condition of below about −80° C., and thus, this method is notsuitable for the mass production.

The 1-halo ribufuranose derivatives may be easily produced under a mildcondition (e.g., room temperature) and reacted with an anionicnucleobase to carry out glycosylation (See U.S. Pat. No. 5,744,597 andEP Patent No. 577304). However, the glycosylation using a 1-haloribofuranose derivative is non-stereoselective (i.e., anomerization at1-position is occurred), leading to a mixture of α- and β-nucleosidesand ultimately to a low yield of the desired β-nucleoside.

U.S. Pat. No. 5,223,608 discloses a process for selectively isolatingthe β-anomer of cytidinenucleoside from a 1:1 mixture of α- andβ-cytidinenucleoside anomers by converting the mixture into thehydrochloride form, dissolving the hydrochloride mixture in hot water,adjusting pH of the resulting solution to 8.2, and cooling and filteringthe solution. However, this process also give a low yield of theβ-anomer.

The present inventors have endeavored to overcome the problems of theprior arts and found that an anomerization is effectively suppressed byremoving the halide compound as it is generated during the glycosylationwhen 1-halo ribofuranose derivative is used and consequently thestereoselectivity can be markedly enhanced.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to providean improved method for preparing 2′-deoxy-2′,2′-difluorocytidine in ahigh purity and yield under a new stereoselective glycosylation reactionusing a 1-halo ribofuranose.

In accordance with the present invention, there is provided a method forpreparing 2′-deoxy-2′,2′-difluorocytidine of formula (I), whichcomprises the steps of

-   -   (i) reacting a 1-halo ribofuranose compound of formula (III)        with a nucleobase of formula (IV) in a solvent to obtain a        nucleoside of formula (II) while continuously removing the silyl        halide of formula (V) produced during the reaction; and    -   (ii) deprotecting the nucleoside of formula (II) to obtain        2′-deoxy-2′,2′-difluorocytidine of formula (I):        wherein,

R is alkyl;

P¹ is a hydroxy-protecting group;

P² is an amino-protecting group; and

X is halogen.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of the invention taken inconjunction with the following accompanying drawings, which show:

FIGS. 1 to 3: high pressure liquid chromatography (BIPLC) scans of thecompounds prepared in Example 4, Comparative Examples 1 and 2,respectively.

DETAILED DESCRIPTION OF THE INVENTION

The inventive method is characterized that the compound of formula (I)can be efficiently prepared by continuously removing the silyl halide offormula (V) which is produced during the glycosylation.

The term “anomer-enriched” used herein means an anomer mixture having aspecific anomer content of greater than 50%, including a substantiallypure anomer. Also, the term “anomerization” means that a substantiallypure anomer or a mixture of α-anomer and β-anomer is epimerized at theC₁-position of a ribofuranose.

The term “carrier” used herein means a solvent that is used to removethe silyl halide produced during the glycosylation and the term “heatingmedium” means a solvent of a high boiling point that can provide asufficient heat to a reaction system and maintain the reaction mixtureat a sufficiently high temperature to enable the continuous removal ofthe silyl halide by distillation.

The term “substituted” used herein means substitution alone or incombination by at least one or more of the groups selected fromhydrogen, cyano, halo, carboalkoxy, toluoyl, nitro, alkoxy and alkyl.

In accordance with the present invention, the stereoselectiveglycosylation is carried out as shown in Reaction Scheme 2.

Specifically, an a-anomer enriched 1-halo ribofaranose of formula (III)is reacted with a nucleobase of formula (IV) for glycosylation toproduce a β-nucleoside of formula (II) together with a silyl halide offormula (V) which may function as a halide source to bring about theanomerization of a-anomer. Accordingly, the silyl halide is continuouslyremoved as it is formed by simple distillation or by using an inert gasuntil the glycosylation reaction is completed. As a result, the extentof anomerization is remarkably reduced and highly stereoselectiveglycosylation occurs in favor of the β-anomer.

The distillation is carried out with simultaneously adding a carrier ora mixture of a carrier and a heating medium which have a high boilingpoint dropwise to the reaction mixture for glycosylation.

Alternatively, the inert gas is passed through a separate tube which isinserted in a reactor to exhaust the silyl halide out of the reactionmixture without affecting the glycosylation reaction. The inert gas isintroduced from the tube which is set up within (bubbling) or above(sweeping) the reacting solution for the removal of silyl halide.

The α-anomer enriched 1-halo ribofuranose of formula (III) used as astarting material in the inventive method has a hydroxy-protectinggroup, and can be prepared by the method described in Korean PatentApplication No. 2004-59623. Exemplary hydroxy-protecting groups areformyl, acetyl, substituted acetyl, propionyl, butynyl, pivalamido,benzoyl, biphenylcarbonyl, substituted biphenylcarbonyl, ethoxycarbonyl,t-butoxycarbonyl, benzyloxycarbonyl, phenoxycarbonyl, benzyl,diphenylmethyl, triphenylmethyl, t-butyl, tetrahydropyranyl, allyl,N-phenylcarbamate, N-imidazoyl carbamate, trialkylsilyl,isopropyldialkcylsilyl, alkyldiisopropylsilyl, triisopropylsilyl andt-butyldialkylsilyl. Among these, benzoyl, biphenylcarbonyl andsubstituted biphenylcarbonyl are more preferred.

The nucleobase of formula (IV) has an amino-protecting group, and it canbe prepared by use of the methods described in U.S. Pat. Nos. 5,371,210,5,401,838, 5,426,183, 5,594,124 and 5,606,048 and EP Patent No. 577303.Exemplary amino-protecting groups are silyl groups such astrimethylsilyl, triisopropylsilyl, tributylsilyl, t-butyldimethylsilyland t-butyldiarylsilyl; carbamates such as t-butoxycarbonyl,benzyloxycarbonyl, 4-methoxybenzyloxycarbonyl and4-nitrobenzyloxycarbonyl; formyl, acetyl, benzoyl and pivaloyl,methoxymethyl, t-butyl, benzyl and tetrahydropyranyl. Among these,trimethylsilyl is most preferred.

In the inventive method, the nucleobase of formula (IV) is used in anamount ranging from 5 to 50 molar equivalents, preferably 10 to 30 molarequivalents, more preferably 15 to 20 molar equivalents, based on the1-halo ribofuranose of formula (III).

The solvents suitable for use in the present glycosylation process arebenzene, substituted benzene, toluene, xylene, decalin, diglyme,2-ethoxyethyl ether, diphenylether, substituted diphenylether, biphenyl,substituted biphenyl, C₆₋₁₄ alkane, substituted C₆₋₁₄ alkane and amixture thereof. Amone these, toluene, C₇₋₁₄ alkane, diphenylether and amixture thereof are preferred, and a mixture of diphenylether andheptane is most preferred. The solvent is used in an amount ranging from5 to 50 ml, preferably 10 to 20 ml based on 1 g of 1-halo ribofuranoseof formula (III).

The carrier used to assist the removal of the silyl halide of formula(V) by distillation must be inert under the glycosylation reactionconditions and preferably has a boiling point higher than that of thesilyl halide. The carrier may be benzene, substituted benzene, toluene,xylene, C₆₋₁₄ alkane, substituted C₆₋₁₄ alkane and a mixture thereof.Among these, toluene, heptane, octane and nonane are preferred, andheptane is most preferred. The carrier is used in an amount ranging from50 to 1000 ml, preferably 100 to 300 ml based on 1 g of the 1-haloribofuranose of formula (III).

In the inventive method, a heating medium having a high boiling point of200° C. or higher may be further used in the form of a mixture with thecarrier, so as to provide a reaction system with sufficient heat andcomplement the loss solvent due to distillation. The heating medium mustbe inert under the glycosylation reaction conditions and preferably hasa boiling point higher than that of the carrier. The heating medium maybe selected from the group consisting of decalin, diphenylether,substituted diphenylether, biphenyl, substituted biphenyl and a mixturethereof. Among these, diphenylether is most preferred. The heatingmedium is used in an amount ranging from 0.1 to 5 vol %, preferably 0.5to 3 vol % based on the amount of the carrier.

It is preferred that the carrier and the heating medium are continuouslyadded to the reaction mixture in a constant rate until the glycosylationreaction is completed, so as to obtain a uniform stereoselectivity.

In addition, a silyl source such as N,O-bis(trimethylsilyl)acetamide(BSA) may be further added in the form of a mixture with the carrier tothe reaction mixture, so as to enhance the removal of the silyl halideby distillation. The silyl source may be used in an amount ranging from0.05 to 1.5 vol %, preferably 0.1 to 0.5 vol % based on the amount ofthe carrier.

In the present invention, an inert gas such as nitrogen, helium, neonand argon, preferably nitrogen, may also be used in the removal of thesilyl halide of formula (V). The inert gas is preferably introduced at aflow rate of 1 l/min or more based on 100 g of 1-halo ribofuranosecompound of formula (III). When the inert gas is introduced at a flowrate less than 1 l/min, the ratio of β-nucleosides to α-nucleosidesbecomes not more than 3.

The glycosylation according to the present invention is carried out at atemperature ranging from 80 to 300° C., preferably 100 to 200° C., morepreferably 130 to 150° C. for 4 to 24 hours.

The progress of the glycosylation may be checked by thin layerchromatography (TLC), ¹H nucleus magnetic resonance (¹H-NMR) or highpressure liquid chromatography (HPLC).

The deprotection of the β-anomer enriched nucleoside of formula (II) maybe carried out by a conventional method. For example, most silylprotecting groups are easily cleaved by the action of water or analcohol. The acyl-amino protecting groups such as formyl, acetyl,pivaloyl and benzoyl are removed by hydrolysis with a strong base. Suchbases include alkali metal hydroxides such as sodium or potassiumhydroxide; alkali metal alkoxides such as sodium methoxide or potassiumt-butoxide; diethylamine, hydroxylamine, ammonia, hydrazine and thelike, among these, ammonia is preferred. Also, the acyl protectinggroups can be removed using an acid catalyst such as methanesulfonicacid, hydrochloric acid, hydrobromic acid, sulfuric acid, or an acidicion exchange resin.

β-Anomer enriched nucleoside of formula (II) may be obtained in a pureform by a separation based on solubility difference from a mixture ofP-anomer enriched nucleoside of formula (II) and the unreacted cytosineas produced after the deprotection. The separation is preferably carriedout by using the solvent system consisting of methylene chloride andmethanol wherein β-anomer enriched nucleoside of formula (II) is highlysoluble while the unreacted cytosine is sparingly soluble.

Thus, in accordance with the stereoselective glycosylation of thepresent invention, a β-enriched nucleoside product having an α:β ratioof 1:4 to 1:14 is obtained.

The β-nucleoside of formula (I) can be isolated in the form ofhemihydrate or dihydrate in a high purity of 99.8% or more and a yieldof 70% or more by a single recrystallization procedure which comprisesdissolving the α/β anomer mixture in water, heating the mixture to atemperature of 40 to 60° C., cooling to 10 to 25° C. and filtering thesolids precipitated during the cooling step. This procedure may beconducted with stirring when the hemihydrate form is derived or withoutstirring for the dihydrate form.

It has been proved that the hemihydrate or dihydrate form of theβ-nucleoside obtained by the present invention is stable for themoisture content changes thereof under the conditions shown in Table 1.TABLE 1 Moisture content (%) Hemihydrate Dihydrate Air  1 day 3.6 11.6 7 days 3.7 11.8 14 days 3.4 11.7 40° C. under  1 day 3.7 12.1 75%relative  7 days 3.8 11.9 humidity 14 days 3.8 11.7Theoretical moisture content of Gemsitabine: Hemihydrate 3.3% dihydrate12.0%

The highly pure hemihydrate or dihydrate of β-nucleosides can bedirectly used without further purification to prepare a pharmaceuticallyacceptable hydrochloride salt of the purity range described in pp892-894 of U.S. Pharmacopoeia (2004).

Accordingly, the present invention also provides a method for preparing2′-deoxy-2′,2′-difluorocytidine hydrochloride comprising reacting2′-deoxy-2′,2′-difluorocytidine of formula (I) or a hemihydrate ordihydrate thereof with hydrochloric acid in an organic solvent.

The present invention will be described in further detail with referenceto Examples. However, it should be understood that the present is notrestricted by the specific Examples.

In Examples, —OCOBiPh or BiPhOCO— structurally means

Also, each product obtained was analyzed by HPLC under two conditions:(1) Zorbax RX-C8 column (4.5×250 mm, 5 μm), NaH₂PO₄.H₂O 13.8 g/H₂PO₄ (pH2.4-2.6) 2.5 me dissolved in 1 l of water for the compound of formula(I); and (2) YMC hydrosphere C18 column (4.6×150 mm, 5 μm), a mixture of760 ml of methanol and 240 ml of NaH₂PO₄.H₂O 13.8 g/H₂PO₄ (pH 2.4-2.6)2.5 ml dissolved in 1 l of water for the compound of formula (II).

EXAMPLE Preparation 1: Preparation of1-α-bromo-2′-deoxy-2′,2′-difluoro-D-ribofuranosyl-5-benzoyl-3-(4-phenyl)benzoateStep 1: Preparation of2′-deoxy-2′,2′-difluoro-D-ribofuranosyl-5-benzoyl-3-(4-phenyl)benzoate

13.5 g of lithium tri-tert-butoxyaluminohydride was dissolved in 160 mlof teterahydrofuran, stirred at room temperature for 30 minutes andcooled to −40° C., to which 20 g ofD-erythro-2-deoxy-2,2-difluoro-pentofuranos-1-ulose-5-benzoyl-3-(4-phenyl)dissolved in 80 ml of teterahydrofuran was added. The mixture was slowlywarmed to room temperature and allowed to react at that temperature for2 hours. Upon completing the reaction, 220 ml of 1N—HCl was added to thereaction mixture and the teterahydrofuran layer was separated. Theaqueous layer was extracted with 220 ml of ether, combined with thepre-separated teterahydrofuran layer, washed successively with 220 mlportion of water, saturated sodium bicarbonate and brine, dried overmagnesium sulfate and filtered. The solvent was removed under a reducedpressure and the residue was purified by silica gel columnchromatography to obtain 18.3 g of the title compound (yield: 91%) as alight yellow syrup.

¹H-NMR (300 MHz, CDC₁₃, δ); 3.89-3.91 (d, 1H), 4.61-4.81 (m, 2H),5.31-5.92 (m, 2H), 7.26-7.70 (m, 10H), 8.05-8.16 (m, 4H)

Step 2: Preparation of2′-deoxy-2′,2′-difluoro-D-ribofuranosyl-5-benzoyl-3-(4-phenyl)benzoate-1β-diphenylphosphate

18.3 g of2′-deoxy-2′,2′-difluoro-D-ribofuranosyl-5-benzoyl-3-(4-phenyl)benzoateobtained in Step 1 was dissolved in 146 ml of toluene, 6.7 ml oftriethylamine was added thereto, and 12.4 ml of diphenyl chlorophosphatediluted in 37 ml of toluene was added dropwise thereto. After 4 hours,48 ml of 1N HCl was added to the reaction mixture to neutralize residualtriethylamine, the toluene layer was separated and the aqueous layer wasextracted with 48 ml of ether. The ether extract was combined with thepre-separated toluene layer and washed successively with water,saturated sodium bicarbonate and brine. The organic layer was separated,dried over magnesium sulfate and filtered. The solvent was removed undera reduced pressure to obtain a mixture of α- and β-phosphate as a solid.The mixture was examined by 1H -NMR analysis to find that theα-phosphate:β-phosphate ratio was 1:10.6. The β-phosphate wasselectively recrystallized from a 3:1 (v/v) mixture of isopropanol andwater, to obtain 26.5 g (yield: 87%) of the title compound as a whitesolid.

¹H-NMR (300 MHz, CDC₁₃, δ); 4.56-4.25 (m, 3H), 5.80 (m, 1H), 5.95 (t,1H), 7.44-6.98 (m, 16H), 7.51 (d, 2H), 7.57 (d, 2H), 7.89 (d, 2H), 8.01(d, 2H)

m.p: 101-103° C.

HPLC purity (area %) : α-phosphate anomer 1.76%, β-phosphate anomer98.24%

Step 3: Preparation of1-α-bromo-2′-deoxv-2′,2′-difluoro-D-ribofuranosul-5-benzoyl-3-(4-phenylbenzoate

22.8 g of2′-deoxy-2′,2′-difluoro-D-ribofaranosyl-5-benzoyl-3-(4-phenyl)benzoate-1β-diphenylphosphateobtained in Step 2 was added to 80.5 ml of 30% HBr/acetic acid and themixture was allowed to react at room temperature for 6 hours. Theresulting solution was diluted with 400 ml of methylene chloride and 500ml of ice water was slowly added thereto. The aqueous layer was removedand the methylene chloride layer was washed successively with ice water,saturated sodium bicarbonate and brine. The methylene chloride layer wasdried over magnesium sulfate and filtered. The filtrate was concentratedunder a reduced pressure to obtain a mixture of α- and β-isomers as asolid. The mixture was examined by 1H -NMR analysis to find that thea-bromo : P-bromo ratio was 10.7:1. The β-bromo compound was selectivelyrecrystallized from isopropanol to obtain 17.0 g (yield: 82%) of thetitle compound as a white solid.

¹H-NMR (300 MHz, CDC₁₃, δ); 8.19 (d, 2H), 8.06 (d, 2H), 7.73 (d, 2H),7.63 (d, 2H), 7.64-7.41 (m, 6H), 6.56 (d, 1H), 5.60 (dd, 1H)

m.p: 111-112° C.

HPLC purity (area %): α-bromo anomer 99.74%, β-bromo anomer 0.26%

Preparation 2: Preparation of1-α-bromo-2′-deoxy-2′,2′-difluoro-D-ribofuranosyl-3,5-di-(4-phenyl)benzoateStep 1: Preparation of2′-deoxy-2′,2′-difluoro-D-ribofuranosyl-3,5-di-(4-phenyl)benzoate

8.66 g of lithium tri-tert-butoxyaluminohydride was dissolved in 120 mlof teterahydrofuran, stirred at room temperature for 30 minutes andcooled to −40° C., to which 15 g ofD-erythro-2-deoxy-2,2-difluoro-pentofuranos-1-ulose-3,5-di-(4-phenyl)dissolved in 100 ml of teterahydrofuran was slowly added. The mixturewas then heated to room temperature and allowed to react for 1 hour. 142ml of 1N-hydrochloric acid was slowly added dropwise to the reactionmixture to decompose excess lithium tri-tert-butoxyaluminohydride, andthe organic layer was separated. The aqueous layer was extracted with150 ml of ether, combined with the pre-separated organic layer, washedsuccessively with 220 ml of water, saturated sodium bicarbonate andbrine, dried over magnesium sulfate and filtered. The solvent wasremoved under a reduced pressure and the resulting solid wasrecrystallized from toluene to obtain 13.4 g of the title compound(yield: 89%) as a white solid.

¹H-NMR (300 MHz, CDC₁₃, δ); 3.45 (s, 1H), 4.85-4.50 (m, 3H), 5.8-5.4 (m,2H), 7.49-7.43 (m, 6H), 7.71-7.61 (m, 8H), 8.18-8.12 (m, 4H)

m.p: 156-158° C.

Step 2: Preparation of2′-deoxy-2′,2′-difluoro-D-ribofaranosyl-3,5-di-(4-phenyl)benzoyl-1β-diphenylphosphate

13 g of 2′-deoxy-2′,2′-difluoro-D-ribofuranosyl-3,5-(4-phenyl) benzoateobtained in Step 1 was dissolved in a mixture of 130 ml of toluene and100 ml of methylene chloride, and 5.1 ml of triethylamine was addedthereto. 7.6 ml of diphenyl chlorophosphate was added dropwise to themixture at room temperature. After 5 hours, the solvent was removedunder a reduced pressure, the resulting solid was dissolved in 130 ml ofmethylene chloride, and 65 ml of 1N HCl was added thereto. The organiclayer was separated, washed successively with water, saturated sodiumbicarbonate and brine, dried over magnesium sulfate and filtered.

The solvent was removed under a reduced pressure to obtain a mixture ofα- and β-phosphate as a solid. The mixture was examined by ¹H-NMRanalysis to find that the α-phosphate:β-phosphate ratio was 1:10.8. Theβ-phosphate was selectively recrystallized from isopropanol to obtain 15g (yield: 83%) of the title compound as a white solid.

¹H-NMR (300 MHz, CDC₁₃, δ); 4.70-4.40 (m, 3H), 5.90 (m, 1H), 6.08 (t,1H), 7.70-7.08 (m, 24H), 8.15-8.04 (dd, 4H)

m.p: 145-147° C.

HPLC purity (area %): α-phosphate anomer 1.29%, β-phosphate anomer98.71%

Step 3: Preparation of1-α-bromo-2′-deoxy-2′,2′-difluoro-D-ribofuranosyl-3,5-di-(4-phenyl)benzoate

13g of2′-deoxy-2′,2′-difluoro-D-ribofuranosyl-3,5-di-(4-phenyl)benzoyl-1β-diphenylphosphateobtained in Step 2 was added to 83.2 ml of 30% BBr/acetic acid and themixture was allowed to react at room temperature for 7 hours. 50 ml ofice water was slowly added to the reaction solution and the solid formedwas filtered. The filtered solid was a mixture of α- and β-bromo and a¹H-NMR analysis showed that the α-bromo:β-bromo ratio was 10.9:1. Theα-bromo compound was selectively recrystallized from ethanol to obtain8.45 g (yield: 83%) of the title compound as a white solid.

¹H-NMR (300 MHz, CDC₁₃, δ); 4.89-4.22 (m. 3H), 5.62 (dd, 1H), 6.55 (d,1H), 7.73-7.42 (m, 14H), 8.63-8.11 (dd, 4H)

m.p: 151-153° C.

HPLC purity (area %): α-bromo anomer 99.67%, β-bromo anomer 0.33%

Example 11-(2′-Deoxy-2′,2′-difluoro-5-benzoyl-3-(4-phenyl)benzoyl-D-ribofuranosyl-4-aminopyrimidin-2-one

Example 1-1

44.5 g of cytosine, 252 ml of hexamethyldisilazane and 252 mg ofammonium sulfate were mixed and refluxed until the solution becamehomogeneous, which was further refluxed for 1 hour. 200 ml of ethylacetate was added thereto and heated to remove remaining unreactedhexamethyldisilazane. A mixture of 160 ml of heptane and 40 ml ofdiphenylether and 10.4 g of1-α-bromo-2′-deoxy-2′,2′-difluoro-D-ribofuranosyl-5-benzoyl-3-(4-phenyl)benzoateobtained in Preparation 1 were added to the resulting solution. Theresulting mixture was reacted for 8 hours while adding dropwise adiphenylether (40 ml)/heptane (4 l) mixture thereto and at the same timecarrying out distillation with maintaining the reaction temperature at130 to 140° C. This procedure allowed continuous removal oftrimethylsilyl bromide from the reaction mixture during the course ofthe reaction. After completing the reaction, 140 ml of heptane was addedto the reaction mixture. The solution was cooled to 100° C., carefullyquenched with 12 ml of water and stirred at room temperature. The solidformed was filtered and washed with heptane to obtain a mixture of α-and β-nucleoside isomers including unreacted cytosine in the form of awhite solid. The nucleoside mixture was examined by HPLC analysis tofind that the α-nucleoside:β-nucleoside ratio was 1:8.8. The solidcontaining the nucleoside mixture and unreacted cytosine was added to amixture of methylene chloride (200 ml) and methanol (40 ml), refluxedfor 1 hour and filtered to remove cytosine. The filtrate was distilledunder a reduced pressure, isopropylether was added to the residue,filtered and the filtrate was dried with warm wind to obtain 10.8 g(yield: 98%) of the title compound as a white solid.

¹H-NMR (300 MHz, DMSO, d-6, δ); 8.1 (d, 2H), 7.9 (d, 2H), 7.8 (d, 2H),7.7 (d, 2H), 7.6 (d, 2H), 7.5-7.4 (m, 7H), 6.3 (t, 1H), 5.8 (m, 1H), 5.7(d, 1H), 4.7-4.6 (m, 3H)

An anomer ratio (HPLC analysis): α-nucleoside/β-nucleoside=1/8.8

Example 1-2

11.1 g of cytosine, 63 ml of hexamethyldisilazane and 63 mg of ammoniumsulfate were mixed and refluxed for 2 hours. 60 ml of toluene was addedto the resulting mixture and heated to remove remaining unreactedhexamethyldisilazane. A mixture of 40 ml of octane and 20 ml ofdiphenylether and 3.5 g of1-α-bromo-2′-deoxy-2′,2′-difluoro-D-ribofuranosyl-5-benzoyl-3-(4-phenyl)benzoateobtained in Preparation 1 were added to the resulting solution. Theresulting mixture was reacted for 10 hours while adding dropwise adiphenylether (10 ml)/heptane (1 l) mixture thereto and at the same timecarrying out distillation with maintaining the reaction temperature at140 to 150° C. This procedure allowed continuous removal oftrimethylsilyl bromide from the reaction mixture during the course ofthe reaction. After completing the reaction, 50 ml of heptane was addedto the reaction mixture. The solution was cooled to 80 to 100° C.,carefully added dropwise 12 ml of water and the mixture was stirred atroom temperature for 1 hour. The solid formed was filtered and washedwith heptane to obtain a mixture of α- and β-nucleoside isomersincluding unreacted cytosine in the form of a white solid. Thenucleoside mixture was examined by HPLC analysis to find that theα-nucleoside:β-nucleoside ratio was 1:5.6. The solid containing thenucleoside mixture and unreacted cytosine was added to a mixture ofmethylene chloride (70 ml) and methanol (15 ml), refluxed for 1 hour andfiltered to remove cytosine. The filtrate was distilled under a reducedpressure, isopropyl ether was added to the residue, filtered and thefiltrate was dried with warm wind to obtain 3.45 g (yield: 93%) of thetitle compound as a white solid.

H-NMR data was the same as in Example 1-1.

An anomer ratio (HPLC analysis): α-nucleoside/β-nucleoside=1/5.6

Example 1-3

2.23 g of cytosine, 12.6 ml of hexamethyldisilazane and 12.6 mg ofammonium sulfate were mixed and refluxed until the solution becamehomogeneous, which was further refluxed for 1 hour. 200 ml of ethylacetate was added thereto and heated to remove remaining unreactedhexamethyldisilazane. 0.26 g of1-α-bromo-2′-deoxy-2′,2′-difluoro-D-ribofuranosyl-5-benzoyl-3-(4-phenyl)benzoateobtained in Preparation 1 was added to the resulting solution. Theresulting mixture was reacted for 6 hours while addingN,O-bis(trimethylsilyl)acetamide (2 ml)/heptane (200 ml) mixturedropwise and at the same time carrying out distillation with maintainingthe reaction temperature at 125 to 140° C. This procedure allowedcontinuous removal of trimethylsilyl bromide from the reaction mixtureduring the course of the reaction. After completing the reaction, thesolution was cooled to 80° C., carefully added dropwise 1 ml of waterand the mixture was stirred at room temperature for 1 hour. The solidformed was filtered and washed with heptane to obtain a mixture of α-and β-nucleoside isomers including unreacted cytosine in the form of awhite solid. The nucleoside mixture was examined by HPLC analysis tofind that the α-nucleoside:β-nucleoside ratio was 1:14.

Example 1-4

340 g of cytosine, 1.835 l of hexamethyldisilazane and 1.84 g ofammonium sulfate were mixed and refluxed until the solution becamehomogeneous, which was further refluxed for 1 hour. 1.2 l of heptane and500 ml of diphenyl ether were successively added to the resultingsolution to lower the temperature of the solution to 100° C. Next, 100 gof1-α-bromo-2′-deoxy-2′,2′-difluoro-D-ribofuranosyl-5-benzoyl-3-(4-phenyl)benzoate obtained in Preparation 1 was added thereto. The resultingmixture was reacted for 12 hours while inserting a separate tube in thereactor and introducing nitrogen at a flow of 1.0 to 1.3 l/min bysweeping thereto with maintaining the reaction temperature at 140 to143° C. This procedure allowed continuous removal of trimethylsilylbromide from the reaction mixture during the course of the reaction.After completing the reaction, the solution was cooled to 80° C. and 100ml of water was carefully added thereto dropwise. The mixture wasstirred at room temperature for 1 hour. The solid formed was filteredand washed with heptane to obtain a mixture of α- and β-nucleosideisomers including unreacted cytosine in the form of a white solid. Thenucleoside mixture was examined by HPLC analysis to find that theα-nucleoside:β-nucleoside ratio was 1:4.9.

Example 1-5

The procedure of Example 1-4 was repeated except that nitrogen wasintroduced into the tube at a flow rate of 3.0 to 3.5 l/min, to obtain amixture of α- and β-nucleoside isomers including unreacted cytosine inthe form of a white solid. The nucleoside mixture was examined by IPLCanalysis to find that the α-nucleoside:β-nucleoside ratio was 1:6.1.

Example 21-(2′-Deoxy-2′,2′-difluoro-3,5-di-(4-phenyl)benzoyl-D-ribofuranosyl-4-aminopyrimidin-2-one

22.2 g of cytosine, 126 ml of hexamethyldisilazane and 126 mg ofammonium sulfate were mixed and refluxed for 2 hours, and 100 ml ofethyl acetate was added to remove unreacted hexamethyldisilazane bydistillation. 80 ml of heptane, 5.93 g of1-α-bromo-2′-deoxy-2′,2′-difluoro-D-ribofuranosyl-3,5-di-(4-phenyl)benzoateobtained in Preparation 2 and 20 ml of diphenylether were successivelyadded to the resulting solution. The resulting mixture was allowed toreact for 9 hours while adding dropwise 4 l of heptane thereto and atthe same time carrying out distillation with maintaining the reactiontemperature at 130 to 140° C. This procedure allowed continuous removalof trimethylsilyl bromide from the reaction mixture during the course ofthe reaction. After completing the reaction, 160 ml of heptane was addedto the reaction mixture. The solution was cooled to 100° C. and 8 ml ofwater was carefully added dropwise thereto. The solution was stirred atroom temperature and filtered. The solid formed was washed with heptaneto obtain a mixture of α- and β-nucleoside isomers including unreactedcytosine in the form of a white solid. The nucleoside mixture wasexamined by HPLC analysis to find that the α-nucleoside:β-nucleosideratio was 1:5.4. The solid containing nucleoside mixture and theunreacted cytosine was added to a mixture of methylene chloride (200 ml)and methanol (40 ml), refluxed for 1 hour and filtered to removecytosine. The filtrate was distilled under a reduced pressure to obtain4 g (yield: 64%) of the title compound as a white solid.

¹H-NMR (300 MHz, CDC₁₃, δ): 8.74-7.27 (m, 19H), 6.38 (m, 1H), 5.83 (m,1H), 5.78 (d, 1H), 4.78-4.45 (m, 3H)

m.p: 250-255° C.

An anomer ratio (HPLC analysis): α-nucleoside/β-nucleoside=1/5.4

Example 3 2′-Deoxy-2′,2′-difluorocytidine (Compound of formula (I)-1:Gemicitabine)

Example 3-1 2′-Deoxy-2′,2′-difluorocytidine hemihydrate

10.8 g of1-(2′-Deoxy-2′,2′-difluoro-5-benzoyl-3-(4-phenyl)benzoyl-D-ribofuranosyl-4-aminopyrimidin-2-oneobtained in Example 1-1 was added to 86 ml of 7N-ammonia in methanol and216 ml of methanol was further added thereto. The mixture was stirred atroom temperature for 12 hours and the solvent was removed under areduced pressure. 120 ml of water and 80 ml of ethyl acetate were addedto the mixture with stirring. The aqueous layer was separated and theethyl acetate layer was extracted with 40 ml of water. The aqueouslayers were combined, washed with 40 ml of diethyl ether and distilledunder a reduced pressure to remove water. 25 ml of water was added tothe resulting residue, the mixture was heated to 45 to 50° C. todissolve the solid, cooled and stirred at room temperature for 2 hoursto allow the precipitation of a solid. The solid was filtered, washedwith water and acetone and dried with warm wind overnight to obtain 3.99g (yield: 76.9%) of the title compound in the form of pure whitehemihydrate.

Moisture content: 3.4%

¹H-NMR (300 MHz, DMSO d-6, δ); 7.7 (1H, d), 7.39 (1H, d), 6.2 (1H, d),6.1 (1H, t), 5.8 (1H, t), 4.2 (m, 1H), 3.9-3.8 (m, 2H), 3.7 (m, 1H)

m.p.=198-202° C.

HPLC purity (area %): β-anomer—99.97%

-   -   α-anomer—less than 0.02%    -   cytosine—less than 0.01%

Example 3-2 2′-Deoxy-2′,2′-difluorocytidine dihydrate

The procedure of Example 3-1 was repeated except that the solution wascooled without stirring during the precipitation of solid, to obtain4.22 g (yield: 81.3%) of the title compound in the form of pure whitedihydrate.

Moisture content: 11.5%

m.p.=220-224° C.

H-NMR data was the same as in Example 3-1.

BPLC purity (area %): β-anomer—99.98%,

-   -   α-anomer—less than 0.01%    -   cytosine—less than 0.01%

Example 4 2′-Deoxy-2′,2′-difluorocytidine (Compound of formula (I)-2:Gemicitabine)

32.2 g of cytosine and 184 mg of ammonium sulfate were added to 184 mlof hexamethyldisilazane. The mixture was refluxed for 1 hour and 250 mlof heptane was added thereto and heated to 135 to 140° C. to distil offunreacted hexamethyldisilazane. 150 ml of heptane and 10.0 g of1-α-bromo-2′-deoxy-2′,2′-difluoro-D-ribofuranosyl-5-benzoyl-3-(4-phenyl)benzoateobtained in Preparation 1 were added to the resulting solution and then38.7 ml of diphenylether was added thereto. The resulting mixture wasallowed to react for 10 hours while adding dropwise 1.5 l of heptanethereto and at the same time carrying out distillation with maintainingthe reaction temperature at 135 to 140° C. This procedure allowedcontinuous removal of trimethylsilyl bromide from the reaction mixtureduring the course of the reaction. After completing the reaction, 240 mlof heptane was added to the resulting solution and 11.6 ml of water wasslowly added thereto. The solid formed was stirred, filtered, washedwith heptane and dried at room temperature, to obtain a mixture of α-and β-nucleoside isomers including unreacted cytosine in the form of awhite solid. The nucleoside mixture was examined by HPLC analysis tofind that the α-nucleoside:β-nucleoside ratio was 1:6.1 (See FIG. 1).The solid was suspended in 300 ml of methylene chloride and 60 ml ofmethanol solution, and refluxed for 2 hours. The resulting mixture wasfiltered, the filtered solid was washed with a mixture of methylenechloride (150 ml) and methanol (30 ml) and distilled under a reducedpressure, to obtain an α/β mixture of1-(2′-deoxy-2′,2′-difluoro-5-benzoyl-3-(4-phenyl)benzoyl-D-ribofuranosyl-4-aminopyrimidin-2-one.The residue solid was added with 200 ml of methanol and 83 ml of7N-ammonia/methanol solution, and stirred at room temperature overnight.After completing the reaction, the solvent was removed under a reducedpressure, and 80 ml of ethyl acetate and 90 ml of water were added tothe residue. The aqueous layer was separated and the ethyl acetate layerwas extracted with 40 ml of water. The aqueous layers were combined andwashed with 40 ml of ether (×2). The water was distilled off under areduced pressure until water was left in the amount of 5 times based onthe theoretical weight of the desired product, and the residue washeated to 50 to 55° C. and cooled to room temperature with stirring for2 hours to induce the precipitation of a solid. The precipitated solidwas filtered, washed with water and acetone and dried with warm windovernight, to obtain 3.69 g (yield: 72.6%) of the title compound in theform of a pure white crystal.

Moisture content: 3.5%

H-NMR data and melting point were the same as in Example 3-1.

HPLC purity (area %): β-anomer—99.9%,

-   -   α-anomer—less than 0.01%    -   cytosine—less than 0.02%

Example 5 2′-Deoxy-2′,2′-difluorocytidine (Compound of formula (I)-3:Gemicitabine)

24 g of N-acetylcytosine and 126 ml of hexamethyldisilazane and 126 mgof ammonium sulfate were mixed and refluxed for 2 hours. 100 ml ofheptane was added to the mixture and unreacted hexamethyldisilazane wasremoved by distillation. 50 ml of octane and 5 g of1-α-bromo-2′-deoxy-2′,2′-difluoro-D-ribofuranosyl-5-benzoyl-3-(4-phenyl)benzoateobtained in Preparation 1 were added to the resulting solution. Themixture was reacted for 8 hours while adding dropwiseN,O-bis(trimethylsilyl)acetamide(1.8 ml)/heptane(900 ml) solutionthereto and at the same time carrying out distillation with maintainingthe reaction temperature at 135 to 140° C. This procedure allowedcontinuous removal of trimethylsilyl bromide from the reaction mixtureduring the course of the reaction. After completing the reaction, 60 mlof heptane was added to the resulting solution which was cooled to 100°C. and 12 ml of water was slowly added thereto. The solid formed wasstirred at room temperature for 2 hours, filtered and washed withheptane, to obtain a mixture of α- and β-nucleoside isomers includingunreacted cytosine in the form of a white solid. The nucleoside mixturewas examined by HPLC analysis to find that the α-nucleoside:β-nucleosideratio was 1:4.8. The nulceoside mixture was suspended in 108 ml ofmethanol and 45 ml of 7N ammonia/methanol solution, the solvent wasremoved under a reduced pressure, and 50 ml of ethyl acetate and 60 mlof water were added to the residue. The aqueous layer was separated andthe ethyl acetate layer was extracted with 20 ml of water. The aqueouslayers were combined and washed with 40 ml of ether (×2). Water wasdistilled off under a reduced pressure, and 15 ml of water was added tothe residue which was heated to 50 to 55° C. and cooled to roomtemperature with stirring for 2 hours to induce the precipitation of asolid. The precipitated solid was filtered, washed with water andacetone and dried with warm wind overnight, to obtain 32.2 g (yield:63%) of the title compound in the form of a pure white crystal.

H-NMR data and melting point were the same as in Example 3-1.

HPLC purity (area %): β-anomer—99.8%,

-   -   α-anomer—less than 0.02%    -   cytosine—less than 0.02%

Example 6 Hydrochloride of 2′-deoxy-2′,2′-difluorocytidine

Example 6-1

3.5 g of 2′-Deoxy-2′,2′-difluorocytidine hemihydrate (moisture content:3.8%) obtained in Example 3-1 was dissolved in 35 ml of acetone and 1.2ml of concentrated hydrochloric acid was added dropwise thereto. Theresulting mixture was stirred at room temperature for 2 hours. The solidformed was filtered, washed with acetone and dried with warm wind toobtain 3.52 g (yield: 91.9%) of the title compound in the form of a purewhite crystal.

¹H-NMR (300 MHz, DMSO, d6): 9.95 (s, 1H), 8.81 (s, 1H), 8.05 (d, 1H),6.15 (d, 1H), 5.96 (m, 1H), 4.14-4.03 (m, 1H), 3.79 (d, 1H), 3.70-3.51(m, 2H)

m.p: 287-292° C.

Example 6-2

3.5 g of 2′-Deoxy-2′,2′-difluorocytidine dihydrate (moisture content:11.5%) obtained in Example 3-2 was dissolved in 35 ml of acetone and 1.2ml of concentrated hydrochloric acid was added dropwise thereto. Theresulting mixture was stirred at room temperature for 2 hours. The solidformed was filtered, washed with acetone and dried with warm wind toobtain 3.23 g (yield: 91.5%) of the title compound in the form of a purewhite crystal.

H-NMR data and melting point were the same as in Example 6-1.

Comparative Example Preparation of 2′-Deoxy-2′,2′-difluorocytidinewithout the distillation for removing silyl halide Comparative Example 1

32.2 g of cytosine and 184 mg of ammonium sulfate were added to 184 mlof hexamethyldisilazane. The mixture was refluxed for 1 hour and 250 mlof heptane was added thereto and heated to 135 to 140° C. to distil offunreacted hexamethyldisilazane. 150 ml of heptane and 10.0 g of1-α-bromo-2′-deoxy-2′,2′-difluoro-D-ribofuranosyl-5-benzoyl-3-(4-phenyl)benzoateobtained in Preparation 1 were added to the resulting solution and then38.7 ml of diphenylether was added thereto. The resulting mixture wasallowed to react for 10 hours with refluxing and maintaining thereaction temperature at 135 to 140° C. After completing the reaction,240 ml of heptane was added to the resulting solution and 11.6 ml ofwater was slowly added thereto. The solid formed was stirred, filtered,washed with heptane and dried at room temperature, to obtain a mixtureof α- and β-nucleoside isomers including unreacted cytosine in the formof a white solid. The nucleoside mixture was examined by BPLC analysisto find that the α-nucleoside:β-nucleoside ratio was 1:1.4 (See FIG. 2).The solid was suspended in 300 ml of methylene chloride and 60 ml ofmethanol solution, and refluxed for 2 hours. The resulting mixture wasfiltered, the filtered solid was washed with a mixture of methylenechloride (150 ml) and methanol (30 ml) and distilled under a reducedpressure, to obtain an α/β mixture of1-(2′-deoxy-2′,2′-difluoro-5-benzoyl-3-(4-phenyl)benzoyl-D-ribofuranosyl-4-aminopyrimidin-2-one.The mixture was added with 200 ml of methanol and 83 ml of7N-ammonia/methanol solution, and stirred at room temperature overnight.After completing the reaction, the solvent was removed under a reducedpressure, and 80 ml of ethyl acetate and 90 ml of water were added tothe residue. The aqueous layer was separated and the ethyl acetate layerwas extracted with 40 ml of water. The aqueous layers were combined andwashed with 40 ml of ether (×2). The water was distilled off under areduced pressure until water was left in the amount of 5 times based onthe theoretical weight of the desired product, and the residue washeated to 50 to 55° C. and cooled to room temperature with stirring for2 hours to induce the precipitation of a solid. The precipitated solidwas filtered, washed with water and acetone and dried with warn windovernight, to obtain 1.80 g (yield: 35.5%) of the title compound in theform of a pure white crystal.

Moisture content: 3.7%

H-NMR data and melting point were the same as in Example 3-1.

An anomer ratio (HPLC analysis): α-nucleoside/β-nucleoside=1/1.4

Comparative Example 2

32.2 g of cytosine and 184 mg of ammonium sulfate were added to 184 mlof hexamethyldisilazane. The mixture was refluxed for 1 hour and 250 mlof heptane was added thereto and heated to 135 to 140° C. to distil offunreacted hexamethyldisilazane. 10.0 g of1-α-bromo-2′-deoxy-2′,2′-difluoro-D-ribofuranosyl-5-benzoyl-3-(4-phenyl)benzoateobtained in Preparation 1 and 36.3 ml of anisole were added to theresulting solution. The resulting mixture was allowed to react for 10hours with refluxing and maintaining the reaction temperature at 135 to140 ° C. After completing the reaction, 240 ml of heptane was added tothe resulting solution and 11.6 ml of water was slowly added thereto.The solid formed was stirred, filtered, washed with heptane and dried atroom temperature, to obtain a mixture of α- and β-nucleoside isomersincluding unreacted cytosine in the form of a white solid. Thenucleoside mixture was examined by HPLC analysis to find that theα-nucleoside:β-nucleoside ratio was 1:1.3 (See FIG. 3). The solid wassuspended in 300 ml of methylene chloride and 60 ml of methanol, andrefluxed for 2 hours. The resulting mixture was filtered, the filteredsolid was washed with a mixture of methylene chloride (150 ml) andmethanol (30 ml) and distilled under a reduced pressure, to obtain anα/β mixture of1-(2′-deoxy-2′,2′-difluoro-5-benzoyl-3-(4-phenyl)benzoyl-D-ribofuranosyl-4-aminopyrimidin-2-one.The mixture was added with 200 ml of methanol and 83 ml of7N-ammonia/methanol solution, and stirred at room temperature overnight.After completing the reaction, the solvent was removed under a reducedpressure, and 80 ml of ethyl acetate and 90 ml of water were added tothe residue. The aqueous layer was separated and the ethyl acetate layerwas extracted with 40 ml of water. The aqueous layers were combined andwashed with 40 ml of ether (×2). The water was distilled off under areduced pressure until water was left in the amount of 5 times based onthe theoretical weight of the desired product, and the residue washeated to 50 to 55° C. and cooled to room temperature with stirring for2 hours to induce the precipitation of a solid. The precipitated solidwas filtered, washed with water and acetone and dried with warm windovernight, to obtain 1.64 g (yield: 32.3%) of the title compound in theform of a pure white crystal.

Moisture content: 3.5%

H-NMR data and melting point were the same as in Example 3-1.

An anomer ratio (HPLC analysis): α-nucleoside/β-nucleoside=1/1.3

The results of glycosylation and deprotection according to Example 4 andComparative Examples 1 and 2 were summarized in Table 2. TABLE 2

HPLC analysis (area %)

β-anomer α-anomer β/α ratio Total yield Ex. 4 84.91 13.94 6.0 72.6%(3.69 g) Com. 57.60 40.90 1.4 35.5% (1.80 g) Ex. 1 Com. 56.17 42.55 1.332.3% (1.64 g) Ex. 2Retention time of the β-anomer peak: 10.08˜10.09Retention time of the α-anomer peak: 8.23

As can be seen from Table 2, in accordance with the present invention,the β-anomer is produced a much higher yield as compared withComparative Examples 1 and 2.

While the invention has been described with respect to the specificembodiments, it should be recognized that various modifications andchanges may be made by those skilled in the art to the invention whichalso fall within the scope of the invention as defined as the appendedclaims.

1. A method for preparing 2′-deoxy-2′,2′-difluorocytidine of formula(I), which comprises the steps of (i) reacting a 1-halo ribofuranosecompound of formula (III) with a nucleobase of formula (IV) in a solventto obtain the nucleoside of formula (II) while continuously removing thesilyl halide of formula (V) produced during the reaction; and (ii)deprotecting the nucleoside of formula (II) to obtain2′-deoxy-2′,2′-difluorocytidine of formula (I):

wherein, R is alkyl; P¹ is a hydroxy-protecting group; P² is anamino-protecting group; and X is halogen.
 2. The method of claim 1,wherein the removal of the silyl halide in step i) is carried out bydistillation.
 3. The method of claim 1, wherein the nucleobase offormula (IV) used in step i) is in an amount ranging from 5 to 50 molarequivalents based on 1 molar equivalent of 1-halo ribofuranose offormula (III).
 4. The method of claim 1, wherein the solvent used instep i) is selected from the group consisting of benzene, substitutedbenzene, toluene, xylene, decalin, diglyme, 2-ethoxyethyl ether,diphenylether, substituted diphenylether, biphenyl, substitutedbiphenyl, C₆₋₁₄ alkane, substituted C₆₋₁₄ alkane and a mixture thereof.5. The method of claim 1, wherein the silyl halide of formula (V) istrimethylbromide.
 6. The method of claim 2, wherein the distillation iscarried out with simultaneously adding a carrier to the reactionmixture.
 7. The method of claim 6, wherein the carrier is selected fromthe group consisting of benzene, substituted benzene, toluene, xylene,C₆₋₁₄ alkane, substituted C₆₋₁₄ alkane and a mixture thereof.
 8. Themethod of claim 7, wherein the carrier is heptane.
 9. The method ofclaim 6, wherein the carrier is used in an amount ranging from 50 to1000 ml based on 1 g of 1-halo ribofuranose.
 10. The method of claim 6,wherein the carrier is used together with a heating medium orN,O-bis(trimethylsilyl)acetamide (BSA).
 11. The method of claim 10,wherein the heating medium is selected from the group consisting ofdecalin, diphenylether, substituted diphenylether, biphenyl, substitutedbiphenyl and a mixture thereof.
 12. The method of claim 11, wherein theheating medium is diphenylether.
 13. The method of claim 10, wherein theheating medium is used in an amount ranging from 0.1 to 5 vol % based onthe amount of the carrier.
 14. The method of claim 10, whereinN,O-bis(trimethylsilyl)acetamide (BSA) is used in an amount ranging from0.05 to 1.5 vol % based on the amount of the carrier.
 15. The method ofclaim 1, wherein the removal of the silyl halide in step i) is carriedout by passing an inert gas through the reaction mixture.
 16. The methodof claim 15, wherein the inert gas is selected from the group consistingof nitrogen, helium, neon and argon.
 17. The method of claim 15, whereinthe inert gas is introduced in the form of bubbling or sweeping.
 18. Themethod of claim 15, wherein the inert gas is introduced at a flow rateof 1 l/min or more based on 100 g of 1-halo ribofuranose of formula(III).
 19. The method of claim 1, wherein step i) is carried out at atemperature ranging from 80 to 300° C.
 20. The method of claim 1, whichin step ii), further comprising after the deprotection, the steps ofdissolving the nucleoside of formula (II) in the form of an α/β anomermixture in water; heating the resulting solution to a temperature of 40to 60° C.; cooling the solution to a temperature ranging from 10 to 25°C. with or without stirring and without pH-adjustment; and filteringprecipitated solids to obtain 2′-deoxy-2′,2′-difluorocytidine of formula(I).
 21. A method for preparing a hydrochloride salt of2′-deoxy-2′,2′-difluoro cytidine of formula (I), which comprisesreacting 2′-deoxy-2′,2′-difluorocytidine of formula (I) or a hemihydrateor dihydrate thereof with hydrochloric acid in an organic solvent.